AMER. ZOOL., 13:567-590 (1973) .
Comparative Histophysiology of the Pancreatic Islets
AUGUST EPPLE AND THOMAS L. LEWIS
Daniel Baugh Institute of Anatomy, Jefferson Medical College of Thomas Jefferson University, Philadelphia, Pennsylvania 19107
SYNOPSIS. The embryological origin of the islet tissue from a common entodermal anlage
with the exocrine pancreas has been questioned recently. The islet tissue may be of.
neural crest origin, and the ancestral islet cells may have been "taste cells in the gut."
Whether the separation of exocrine and endocrine tissue in the cyclostomes is an
original one or not remains an open phylogenetic key question.
One or more islet hormones affect the exocrine pancreas tissue. However, the islet
topography in various groups shows that intrapancreatic islet dissemination is not a
general prerequisite for the normal function of the exocrine tissue.
The D-cell is now generally recognized as the source of a third islet hormone. A
fourth granular cell type (X-cell) may well secrete a fourth islet hormone. The
significance of the amphiphil islet cells, found in various species, and of the "light"
cells of the cyclostomes requires further studies.
The islet function in lower vertebrates is largely unknown. So far, neither the islet
cytology nor the known effects of pancreatectomy allow far-reaching conclusions. The
evolution of the islet functions may be only understood when their interactions with
the pituitary functions become dear.
docrine and the exocrine pancreas; (2) the
status of islet cells other than A- and Bcells; and (3) the structure of the endocrine
pancreas as related to its function.
INTRODUCTION
During the past decade, enormous progress was made in comparative islet research, and with this progress, new and unexpected problems have arisen. Most of the
pertinent histophysiological data have been
summarized in a number of recent reviews
(Epple, 1968a; Fujita, 1968; Ferner and
Kern, 1969; Gossner, 1969; Kern, 1971;
Falkmer and Patent, 1972; Falkmer and
Marques, 1972; Hellman and Taljedal,
1972; Kern and Griibe, 1973; Lange, 1973).
Comparative aspects of the structural and
functional interrelations between the endocrine cells in the gastrointestinal tract
and in the pancreas will be covered in the
proceedings of a symposium, held in early
1972 (see Fujita, 1973a).
Therefore, instead of repeating the wellcovered facts, this review attempts to point
out some of the current frontier problems in
islet research. In particular, it will deal with
the following topics: (1) the phylogenetic
and functional relationships of the enPersonal investigations, reported in this review,
were supported by fellowships and grants of the
"Deutsche Forschungsgemeinschaft" and by grants
AM-12643 and RR-5414 from the National Institutes
of Health to A. Epple.
THE EMBRYOLOGICAL ORIGIN OF THE TISSUE
Four possible embryological origins of the
islet cells have been suggested in recent publications: (1) an origin from mesodermal
precursors (Wessels, 1968); (2) a neural crest
origin, common with certain anterior pituitary cells, the thyroid C-cells, cells of the
carotid bodies, diffuse bronchial endocrine
cells, chromaffin tissue and gastrointestinal
endocrine cells (Pearse and Polak, 1971);
(3) a common entodermal origin with the
majority of protein secreting cells from a
"vertebrate enterosecretory system," whose
descendents include exocrine and endocrine
cells in very different locations (Adelson,
1971); (4) a common entodermal origin
with the exocrine pancreas, with which the
endocrine cells share a "protodifferentiated"
phase, before a regulatory factor causes a
differentiation into different endocrine and
exocrine cells (Pictet and Rutter, 1972).
While Pictet and Rutter's (1972) view is
in accordance with most morphological
studies before 1968, the hypotheses of Pearse
567
568
AUGUST EPPLE AND THOMAS L. LEWIS
(Pearse and Polak, 1971) and of Adelson
(1971) are supported by molecular-genetic
and clinical data. A common embryological
origin for topographically widely separated
cells with structurally similar secretions,
such as, for example, the ones which produce the nerve growth factor in the ectodermal (!) sublingual gland and the insulin
secreting pancreatic B-cells, is a tempting
thought (Frazier et al., 1972). Ectopic endocrine tumors, secreting a host of hormones of normally very different origins
(see, e.g., Liddle et al., 1969), also favor
the postulate of a common origin of certain
endocrine cells. However, neither experimental (Pearse and Polak, 1971; Pictet and
Rutter, 1972) nor ultrastructural studies
(Like and Orci, 1972) so far provide a
satisfactory answer as to the early embryological origin of the islets.
The proliferation of islet tissue from
small pancreas ductules under certain preand postnatal conditions is beyond any
doubt (see, e.g., Edstrom and Falkmer,
1967, 1968; Boquist and Falkmer, 1970;
Suzuki and Matsuyama, 1971; Boquist,
1972; Edstrom and Boquist, 1973). However, it seems that the embryological origin
of these ductules, or at least of the insulogenic cells of these ductules, requires further studies. There is no evidence that they
can originate from exocrine acini (Boquist
and Edstrom, 1970). Earlier pertinent data
are discussed in the excellent study of
Bencosme (1955).
THK PHYLOCENETICAL ORIGIN OF THE
ENDOCRINE PANCREAS
T h e almost "classical" proposition that
the original islet cells were modified mucosa
cells (Barrington, 1964; Steiner et al., 1969,
1973; Falkmer, 1972) received support from
the description of resorptive cells in the follicles of Langerhans in Myxine (Thomas
and Ostberg, 1972). Additional data on the
fine-grained cells of the intestine of Branchiostoma (Windbladh Biuw, and Hulting,
1971) may be of great importance in this
matter, since it is possible that these cells
have a dual exocrinc-endorrine function.
However, recent studies suggest two further
possibilities (Fig. 1):
1) The ancestral islet cells secreted inducers, which later on became hormones;
this has been suggested for a common
ancestor of both the nerve growth factor
and insulin (Frazier et al., 1972).
2) T h e ancestral islet cells resembled the
"open" type of basal-granulated cell of the
intestine, as described by Fujita and coworkers (Fujita and Kobayashi, 1971, 1973;
Osaka et al., 1971) in the human and dog:
their specialized microvilli reached the intestinal lumen, where they served as pHand/or chemo-receptors, while their granule-containing basal portion was in contact
with a capillary, in which they released
their secretion in response to changes in the
gut content. Later on, these cells lost their
contact with the gut lumen and became
"closed" cells (Kobayashi et al., 1971),
whose secretions now depended on other
stimuli. As pointed out by Fujita (19736),
the "open" basal-granulated cells may be
thought of as "taste cells in the gut"; an
ingenious idea, which may be consistent
with a neural crest origin (Pearse, 1969).
THE RELATIONSHIPS BETWEEN THE ISLET
TISSUE AND THE EXOCRINE PANCREAS
Ontogenic-phylogcnetic relationships
Ontogenic and phylogenetic relationships between exocrine and endocrine pancreas are hardly less debated than the early
embryology of the endocrine pancreas. Yet
proof (or disproof) of a common origin of
both pancreas tissues will be of great importance for the biologist as well as the
clinician. For the biologist, this could answer the question whether the complete
separation of exocrine and endocrine pancreas in the cyclostomes is a primary one,
or a result of a partial "degeneration" of
the exocrine tissue. If the latter alternative
should be true, then the pancreas of certain
chondrichthyes (Bargmann, 1939; Epple,
1968fl, 1969) and of Latimeria (Grossner,
1968) would be the models for the "stem
form" of the pancreas types of higher verte-
COMPARATIVE ISLET HISTOPHYSIOLOGY
FIG. 1. Hypothetical possibilities of the embryological and phylogenetical pancreas development.
A-C, Possible functions of the ancestral islet cells.
//, Exocrine cells secrete enzymes into the gut
lumen, where they are partly split by other enzymes;
the rest chains are resorbed by mucosal cells and
released as hormones into the blood stream, li, The
ancestral islet cells are mucosal chemoreceptors,
which upon stimulation by changes in the intestinal
contents release granules into the blood stream. C,
The ancestral islet cells produce inducer substances
in the embryonic digestive tube. At the present
state of knowledge, these possibilities are not
mutually exclusive for the different types of islet
cells.
Three different ways of islet evolution from a
primitive intestine D with individual exocrine
and endocrine cells: (1). The islets develop independently from the exocrine pancreas and mix
with it secondarily (D-» £-»£/-»./) . (2) . The ancestral islet cells leave together with the evolving extramural pancreas from the intestine
569
(3) . The islet cells originate from the exocrine
pancreas, which at this time is already a specialized
gland (either intramurally or, as shown here for
clarity, extramurally located) (D->G->/-»7). Exocrine cells: big apical granules, light nucleic; endocrine cells: diffuse dark granulation.
A, Hypothesis of Steiner and coworkers. B, Hypothesis of Fujita and coworkers. C, Hypothesis of
Fra/ier and coworkers. D, Hypothetical situation,
where the islet cells have lost contact with the
gut lumen and resemble the "closed" cells of
Fujita and coworkers. E, Lamprey situation. F and
/, Resembling to some extent the situation in the
human embryo, some chondrichthyes and iMtimeria.
G, Not known for the pancreas with certainty,
though some authors claim that the ventral pancreas
anlagan are free of endocrine tissue; typical for
the mammalian parotid gland. H, Similar to the
situation in teleosts with a Brockman body; however, no intrapancreatic endocrine tissue present. / ,
Tetrapod-like pancreas.
570
AUGUST EPPLE AND THOMAS L. LEWIS
brates. For the clinician, a definite disproof
of a common phylogenetic origin would indicate that there is practically no hope for
the induction of neoformation of B-cells
from exocrine tissue in juvenile-type diabetes; a possibility suggested by mixed endocrine-exocrine pancreas cells (Orci et al.,
1970). In order to define the basic possibilities we are facing in this matter, it may be
helpful to outline them as follows (see Fig.
Possibility 1. The islet tissue was originally a part of the diffuse gastrointestinal endocrine system of the invertebrates (see
Falkmer, 1972). Before an extramural exocrine pancreas was formed, the islet tissue
had already become a compact and separate
organ, as in the cyclostomes (see Epple,
1969; Falkmer and Patent, 1972; Falkmer
et al., 19735). When the exocrine pancreas
evolved as a distinct gland, it became mixed
with the endocrine tissue. In this case, the
islet tissue could be of either entodermal
or non-entodermal origin.
In the favor of this possibility is the
similarity and/or identity of the so-called
gastrointestinal A- and D-rells with the synonymous islet elements (for literature, see
Polak et al., 1971; Vasallo et al., 1972;
Fujita and Kobayashi, 1973). However, the
absence of A- and D-cells in the islet organ
of the cyclostomes poses a problem, unless
one assumes that they left the intestinal
wall after B-cells, together with the exocrine pancreas.
The absence of glucagon in the cyclostome islets is now accepted by most investigators (Falkmer and Marques, 1972; van
Noorden et al., 1972; Falkmer et al., 19736).
This is in fairly good agreement with Weinstein's (1968) molecular-genetic calculation
concerning the time when glucagon and
secretin differentiated from a common precursor substance (see Epple, 1969). On the
other hand, Barrington and Dockray
(1970) have described secretin-like activity
in extracts of lamprey intestines and Assan
et al. (see Falkmer and Marques, 1972) report on a glucagon-like immunoreactivity
in gut, liver, and islets of both the lamprey
and hagfish.
Possibility 2. Diffuse, already specialized
endocrine cells left the intestinal wall together with the neighboring mucosal
glands; they "hypertrophied" from single
elements into islets, while the adjacent mucosal glands enlarged and became the exocrine pancreas. Also in this case, the islet
tissue could be of either entodermal or
non-entodermal origin.
This possible way of islet evolution is
suggested by the well-known association of
exocrine and endocrine cells in the glands
of the gastrointestinal tract. Here, it is not
difficult to imagine that, for example, a
part of Brunner's glands left the duodenum to become an extramural pancreas.
The early simultaneous appearance of future exocrine and endocrine elements in the
dorsal pancreas anlage (see, e.g., Siwe, 1926;
Wolf-Heidegger, 1936; McAlpine, 1951;
Drews et al., 1969, Pictet and Rutter, 1972)
may give some support to this possibility.
Possibility 3. Endocrine and exocrine
pancreas both developed from a common
entodermal pancreas anlage, which originally only formed exocrine tissue.
The third possibility represents the classical concept (see Bargmann, 1939). Its best
morphological support is (a) the similarity
between the structure of the islet organ of
the selachians and Latimeria (see Epple,
1969) and certain states of the human embryonic pancreas (Neubert, 1927), and (b)
the existence of mixed endocrine-exocrine
cells (Orci et al., 1970; Pictet and Rutter,
1972).
However, since even electron microscopic
studies have so far not been able to clarify
the origin of the first islet cells of the human
pancreas (Like and Orci, 1972), light microscopic similarities can no longer be accepted
as convincing evidence; similarly, topographical relationships between ducts and
islet tissue, pointed out by many investigators in various species, especially in reptiles (Gabe, 1970; Miller and Lagios, 1970),
are of no special help in this question.
Mixed exocrine-endocrine cells would be
the best argument in favor of a common
embryological origin of both pancreas tissues, if it could be shown that thev de-
COMPARATIVE ISLET HISTOPHYSIOLOGY
veloped from one common precursor cell,
as is often assumed. Unfortunately, in this
matter the interpretations of the electron
microscopists today are as controversial as
the opinions of the histologists during
decades following the publication of Laguesse's "balancement" theory at the turn
of the century (see Bargmann, 1939). Mixed
endocrine-exocrine cells have been reported
in ultrastructural studies for various vertebrate groups: chondrichthyes (Patent,
1971), anurans (Kobayashi, 1969; Lange,
1968), birds (Bjorkman and Hellman,
1964; Dieterlen-Lievre, 1965; Braun-Blanquet, 1969), mammals (see Faller, 1969;
Orci et al., 1970; Brown and Still, 1970).
Some authors consider these cells as artifacts
(Marx et al., 1970; Brinn, 1973), and others
suggest that they result from the fusion of
neighboring cells (Kobayashi, 1969; Petkov,
1971).
Functional relationships
For the histophysiologist, regardless of the
ontogenic and phylogenetic relationships
between the islets and the exocrine pancreas, the question arises as to the possible
functional significance of the intermingling
of both, tissues. Ferner (1957) suggested that
the dissemination of the islets within the
exocrine tissue follows the same functional principle as the dissemination of
the Leydig cells in the testis, i.e., it serves
the local supply of hormones. Henderson
(1969), obviously not aware of Ferner's
pertinent publications, comes to a similar
conclusion and compares the role of the
islet hormones with that of the adrenocortical secretions, which control the medullary adrenaline formation (Wurtman,
1966). One phylogenetic difficulty arises
with this interpretation: Why is the dissemination of islet tissue, well represented
already in the chondrichthyes (see, e.g.,
Thomas, 1940), again abandoned in higher
forms?
In teleosts (Epple, 1969; Brinn, 1973),
the islet tissue is often concentrated within
Brockmann bodies, which are largely
separated from the exocrine tissue. In the
toad, Bufo arenarum, many islets are con-
571
centrated in one particular pancreas process
(von Lawzewitsch, 1963fc>). Several groups
of reptiles show a strong tendency to concentrate the islet tissue in the caudal pancreas (Gabe, 1970); in birds, the splenic
lobe (Gianelli, 1902) consists of a large
amount of islet tissue with very little exocrine parenchyma (Clara, 1924a; and many
others).
Despite these problems arising from some
groups, there can be little doubt that the
endocrine pancreas can actually affect the
surrounding exocrine tissue. It has long
been known that the acini in the immediate
neighborhood of the islets stain differently
from the rest of the exocrine tissue, forming
so-called "zymogen mantles" (for older literature see Hellman et al., 1962). Such
zymogen mantles have been described for
mammals and birds, and we also find them
in the eel, Anguilla rostrata (Brinn and
Epple, unpublished). In duck embryos,
zymogen mantles are already present a considerable time before hatching (Svennevig,
1967). Since the exocrine nuclei at the periphery of both A- and B-islets of the duck
are enlarged, Wallgren and Hellman
(1962) suggested that insulin and one or
two additional islet hormones have a
stimulatory effect on the neighboring exocrine parenchyma.
Epple (19686) found that in pullets, fed
from birth on a special diet, zymogen mantles developed at the periphery of both
types of islets and enlarged continuously
until almost the whole exocrine parenchyma consisted of "mantle tissue." Here, however, the mantle cells were small, contained
small, sometimes even hyperchromatic
nuclei, and were densely filled with very
big granules which showed metachromasia
and UV-fluorescence after staining with
pseudoisocyanin plus preoxidation. Similarly, Braun-Blanquet (1969) found ultrastructurally giant exocrine granules at the
periphery of the B-islets of the duck. She
concludes that these granules developed
by confluence of normal granules.
From these observations, the following
conclusions appear appropriate: (1) Islet
hormones many not only stimulate, but
also inhibit the exocrine pancreas. The rate
572
AUGUST EPPLE AND THOMAS L. LEWIS
of islet secretion, different islet hormones,
or a varying sensitivity of the exocrine tissue
may determine the direction of the effect of
islet hormones. (2) The continuous
spreading of the zymogen mantles from the
islet periphery into distant pancreas regions,
which certainly do not have common capillaries orvenules with the islets, suggests that
the islet hormones are also released into
the intercellular space; this would not be an
unusual route for a one-time inducer substance, a role that has been suggested for
insulin (Frazier et al., 1972). (3) Either the
hormone of the D-cells (which are found in
both types of the chicken islet), or as concluded for the duck, at least two islet hormones affect the exocrine pancreas.
The physiological importance of the islet
secretions for the exocrine pancreas (for
literature, see Henderson, 1969) deserves
further studies. However, it may not be too
far-fetched to predict considerable species
differences due to such factors as a varying
sensitivity of the exocrine tissue to the islet
hormones and the interaction of the islet
hormones with the acinar innervation and
gastrointestinal hormones. The available
data do not show if the local distribution
of islet hormones is the original modus of
islet-exocrine interactions: studies in elasmobranchs may help to clarify this question. On the other hand, it is clear that
portions of the exocrine pancreas of some
species, e.g., the intrahepatic pancreas of
some teleosts (Epple, 1969), can be reached
by the islet hormones only after their passage through, and their dilution in, the
systemic circulation. It also remains a
matter of speculation whether the exocrine
pancreas (or its precursor in the intestine)
was phylogenetically die first target of the
islet secretions, or whether a specialized islet tissue, already otherwise "committed,"
took the evolving extramural pancreas as
an additional target.
THE STATUS OF ISLET CELLS OTHER THAN
A- AND B-CELLS
Despite a large number of recent studies,
the definitions and the nomenclature of the
islet cells remain so confusing that an explanation of the terminology of this paper
appears indispensable. Since different electron microscopic techniques often resulted
in very different interpretations, the definitions of this review are based on the conventional staining techniques. Histochemical and ultrastructural data may be
found in the reviews of Gossner (1969),
Kern (1971), Hellman and Taljedal
(1972), Lacy and Greider (1972), Munger
(1972a), Brinn (1973), and Lange (1973).
In the following, only the D-cells, the
amphiphils, the X-cells (of the holocephalians), and the "light" cells (of the
cyclostomes) will be treated. Agranular or
C-cells, which in some cases may be chromophobic rather than without granules (see,
e.g., Lange, \97la,b; Lacy and Greider,
1972), have been investigated recently in
great detail by Falkmer and co-workers
(see Boquist, 1972). They are reviewed
by Lange (1973) and Brinn (1973). Other
rare islet cells, such as the E-cells (Bencosme and Liepa, 1955; Munger et al.,
1965; Lazarus and Shapiro, 1971), will not
be considered, since only a few data are
available. Occasional gastrointestinal endocrine elements, e.g., enterochromaffin
cells (Parilla et al., 1969), may occur within the pancreas, which is understandable,
considering its embryonic origin (Like and
Orci, 1972; Pictet and Rutter, 1972).
Ultrastructurally, more than three types
of granular islet cells have been found in
various vertebrates: holocephalians (Patent,
1971), teleosts (Brinn, 1973), amphibians
(Kobayashi, 1969; Trandaburu and Ionescu, 1969; Lange, 1971 b), reptiles (Trandaburu and Calugareanu, 1969; Rhoten,
1970), mammals (Hoyos-Guevara, 1969;
Misugi et al., 1970; Munger, 1970; Lazarus
and Shapiro, 1971; Deconinck et al., 1972;
Pictet and Rutter, 1972; Vassallo et al.,
1972). Their identification with the light
microscope is still problematic in some
cases, and their mutual relationships require further studies. Therefore, a detailed
discussion of these elements appears premature. However, considerable information
regarding the histological properties of
573
COMPARATIVE ISLET HISTOPHYSIOLOGY
these cells has been obtained and is presented below and in Table 1.
m
Histological criteria of the islet cells
B-cells. This is the only islet cell whose
staining characteristics seem to be almost
agreed upon. The B-cells stain after oxidation with aldehyde fuchsin (Scott, 1952;
Gabe, 1953; and others) and aldehyde
thioin (Paget, 1954). When pseudoisocyanin is applied after oxidation, they
also stain mtetachromatically and give UVfluorescence (Epple, 1967a). They are not
acidophilic and are not argyophilic with
the methods of Hellman and Hellerstrom
(1960) and Grimelius (1968).
Since another type of islet cell, the amphiphil (Epple, 19676) shares with the Bcells the stainability with aldehyde fuchsin
and pseudoisocyanin, the alleged specificity
of both methods for insulin requires some
consideration. According to Schiebler and
Schiessler (1959), the pseudoisocyanin reaction in the pancreas is specific for insulin.
However, a pseudoisocyanin reaction in the
exocrine pancreas tissue of selachians (Ferner and Kern, 1964; Epple, 19676), toads
(Epple, 1966a), and chickens (Epple,
19686) shows that additional pancreas substances react as well. In the shark, it could
clearly be shown that the rim, and not the
core, of the exocrine granules is pseudoisocyanin-positive (Epple, 19676). Careful
focussing in species with large B-cell granules (sharks, amphibians) revealed that
again only the granule rim was stained
metachromatically with pseudoisocyanin
(Epple, unpublished). This corroborates the
findings of H. Fujita and Matsuno (1967)
who observed ultrastructurally in the hypoglycemic phase of alloxan diabetes the persistence of the granule rim when, due to insulin release, the core material was no
longer visible, but the cells still were aldehyde fuchsin-positive. The problems of correlating B-cell staining with the insulin
content are also discussed by Chan and
Fontaine (1971) and Wellman et al. (1971).
Tn agreement with Lazarus and Barden
(1961) and Fujita et al. (1968), we believe
CO
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574
AUGUST EPPLE AND THOMAS L. LEWIS
that the rim of the granules, and most
likely the rim only, reacts with pseudoisocyanin and aldehyde fuchsin. As pointed
out previously (Epple, 19676), in vitro
experiments with insulin (Schirner, 1959;
Kvistberg et al., 1966; Rothwell and Fielding, 1970) can only show possibilities, but
never guarantee that in the histological
section the same reaction takes place. On
the other hand, Fujita and Kobayashi
(1973) suggest in a recent paper that the
substance reacting with pseudoisocyanin
after oxidation is identical with the metachromatic material found in the cells of
Pearse's (1969) "APUD" (Amine Precursor
Uptake and Decarboxylation) series. This
substance, possibly a lipoprotein, stains
with toluidin blue and pseudoisocyanin
after pretreatment with HC1. Thus, it seems
that the only method generally acceptable
as specific for insulin is the immunofluorescence technique (Arnold et al., 1972),
though the discovery of other pancreas
substances cross-reacting with insulin antibodies, if it happens, should not come as
a surprise. The close relationship of the
exocrine and_ endocrine secretions of the
digestive tube (Track, 1969; Adelson, 1971;
Rieser, 1967; and others) has to be kept in
mind. However, there is no disagreement
concerning the origin of insulin in the Bcells.
A-cells. These cells, called by Hellman
and co-workers ao-cells (see, e.g., Hellman
and Taljedal, 1972), contain granules whose
acidophilia varies with the species (see,
e.g., Epple, 1967a). At least in a number of
species, the phosphotungstic acid hematoxylin method of Levene and Feng (1964) and
the Azan modification of Gomori (1939)
stain the A-granules selectively. The A-cells
are argyrophilic with the method of Grimelius (1968), though this method also stains
a fraction of islet cells which are argyrophilic with the method of Hellman and
Hellerstrom (1960). Though it has been
assumed that the A-cells are negative with
the latter method, this seems not to be true
in some reptiles (Gabe, 1970). The A-cells
share with the D-cells the stainability with
lead-hematoxylin (Solda et al., 1969). The
toluidin blue methods of Manocchio (1960,
1964) and their modification (Solcia et al.,
1968; Gabe, 1970) allow in many species the
separation of the A-cells from the D-cells,
which become metachromatic, while the
A-cells remain orthochromatic. The A-cells
are not stained with pseudoisocyanin or
aldehyde fuchsin after oxidation. They are
generally recognized as the source of glucagon (see Falkmer and Marques, 1972;
Falkmer and Patent, 1972; Lange, 1973).
D-cells. They are argyrophilic with the
method of Hellman and Hellerstrom (1960)
who prefer to call them ai-cells (see Hellman and Taljedal, 1972). In some species
they are metachromatic with simple
toluidin blue staining (Manocchio, 1960;
Epple, 1966£>), in others the metachromasia
shows only after methylation and demethylation (Manocchio, 1964), or after acid
hydrolysis (Solcia et al., 1968). They are
metachromatic and give UV-fluorescence
with pseudoisocyanin after methylation and
demethylation (Epple, 1967a) as well as
after HC1 treatment (Solcia et al., 1968).
They are not stained with pseudoisocyanin
after oxidation. In many species, they show
a weak to moderate basophilia. At the present state of knowledge, the best means
for their identification may be their metachromatic staining with toluidin blue or
pseudoisocyanin after acid hydrolysis, and
their negative reaction with pseudoisocyanin after oxidation. By the first two
methods, D-cells should be distinguished
from the A-cells and probably also from the
X-cells (Patent and Epple, 1967), and by
the other method from amphiphils (Epple,
19676).
X-cells (as identified in the holocephalians by Fujita [1962] and by Patent and
Epple [1967]). The name X-cells was applied originally to the A-cells of the horse
(see Ferner, 1952), but then also to a
special cell type of the dog (Bencosme
and Liepa, 1955), to intrainsular neurosecretory fibers (Trandaburu and Calugareanu, 1969), and to a cell type of the
human pancreas very numerous in the
Verner-Morrison syndrome (Creutzfeldt et
al., 1970). With certainty, the X-cell re-
COMPARATIVE ISLET HISTOPHYSIOLOGY
ferred to here has only been identified in
the holocephalians, where it amounts to
about 50% of the islet cells. It is acidophilic,
but less than the A-cells of these fish, and it
differs in shape and topography from the
D-cells with which it shares argyrophilia
after Hellman and Hellerstrom (1960).
However, contrary to the D-cells of the holocephalians, it is not argyrophilic after Palmgren's (1948) silver impregnation (Fujita,
1964). The X-cells do not stain with aldehyde fuchsin or pseudoisocyanin after oxidation.
Amphiphih. Amphiphil islet cells (Epple,
19676) are argyrophilic with the method
of Hellman and Hellerstrom (1960), but
also stain with pseudoisocyanin or aldehyde fuchsin after oxidation. In most cases,
though not in all, the pseudoisocyaninmetachromatic material after oxidation
shows a bright fluorescence when UV-light
is applied.
Another type of "amphiphils" has been
described by Khanna and Mehrotra (1968)
and Hirano and Honma (1971) in teleosts.
These cells are not argyrophilic, but stain
with aldehyde fuchsin, as well as with
azocarmin.
The D-cells
The postulate that the D-cells secrete a
third islet hormone (Epple, 19616; Fujita,
1964) appears now generally accepted. Apart
from various histophysiological (Epple,
1963, 1965, 1968a; Fujita, 1968), histochemical (Solcia and Sampietro, 1965; Solcia et
al., 1968), and ultrastructural (see, e.g.,
Greider et al., 1970; Wellman et al., 1971;
Munger, 1972a; Brinn, 1973) characteristics,
the best evidence of the functional autonomy of this cell type is the electron microscopic observation of a mitosis in a granulecontaining D-cell (Like and Orci, 1972).
However, the nature and function of the Dcell hormone are still a matter of discussion, one of the main reasons for this being
the difficulty of identifying the normal
equivalent of the gastrin-producing tumor
cells of the Zollinger-Ellison syndrome (see
Munger, 1972a; Vassallo et al., 1972). According to Lechago et al. (1971), "the gas-
575
trin producing cells of the dog antrum
resemble the F-cells of the uncinate process
of the pancreas and differ from the pancreatic D-cells."
While Lomsky et al. (1969), Greider and
McGuigan (1971), and Vassallo et al. (1972)
report an immunohistological reaction of
gastrin antibodies within the D-cells, others
obtained negative results (Creutzfeldt et
al., 1971; Deconinck et al., 1972). However,
Greider and McGuigan (1971) report the
presence of small amounts of gastrin-like
immunoreactivity in pancreas extracts, and
Hellman and Lernmark (1970) found "that
a water extract of pigeon ai-cells significantly reduced the amounts of insulin secreted
in vitro"; they conclude that the inhibitor
of insulin release in the pancreatic ax-cells
(=D-cells) may be identical with gastrin.
An argument against the production of
gastrin by the pancreatic D-cells is their
ultrastructural difference from the pyloric
G-cells, which are generally recognized as
the source of gastrin (see, e.g., Forssmann
and Orci, 1969; Solcia et al., 1969; Pearse
and Bussalotti, 1970; Vassallo et al., 1972;
Fujita and Kobayashi, 1971, 1973). Furthermore, attempts to extract gastrin from normal pancreas for bioassays failed (McGuigan, 1972; Blair et al., 1969). Gastrin
seems to be absent in the chicken, since
it could not be detected in the gastrointestinal tract of this species, and the dose required for the maximal stimulation of gastric secretion in the chicken was 65 times
higher than in the human (Ruoff and
Sewing, 1970). If further studies should
confirm the absence of gastrin also for the
chicken pancreas, which contains many Dcells (Hellman and Hellerstrom, 1960;
Mikami and Mutoh, 1971), then it should
be finally clear that gastrin is not produced
in the normal D-cells.
It appears possible that the D-cells contain a hormone which is immunologically
very similar to gastrin, but otherwise different. The immunological similarities between gastrin and cholecystokinin-pancreozymin and between glucagon and "enteroglucagon," and the structural similarities between glucagon and secretin (see
Creutzfeldt, 1970; Lefebvre and Unger,
576
AUGUST EPPLE AND THOMAS L. LEWIS
1972) suggest this possibility.
On the other hand, there are several substances which could turn out to be islet
hormones. Since the older literature on this
matter has been compiled in a previous
paper (Epple, 1965), only recent studies
will be reviewed here.
Power and his colleagues (Power, 1967;
Power et al., 1967) postulate that a substance with insulin-like activity is a third
pancreatic hormone. This substance was
found in the serum of normal individuals,
where it increased in concentration after
both tolbutamide and intravenous glucose.
A patient with an islet-cell adenoma produced high serum concentrations of this
substance, which were reduced to subnormal levels by excision of this tumor.
This substance has a molecular weight of
70,000-150,000, far above insulin; it cannot
be neutralized by antibodies to insulin, and
when subjected to conditions that separate
insulin into its A- and B-chains, no comparable fractions are found.
Sutton and Taghizadeh (1969) postulate
a gluconeogenic islet hormone, whose absence in pancreatectomized and partially
hepatectomized rats causes a fatal hypoglycemia, which cannot be prevented by
glucagon injections.
Since the original description of an endocrine pancreas tumor associated with
watery diarrhea and hypokalemia (Verner
and Morrison, 1958), various-reports have
confirmed the existence of this syndrome
(see Munger, 1972a). It appears possible
that the endocrine factor involved is also
present in the normal pancreas. However,
hyperplastic islets of a patient with this
Verner-Morrison syndrome contained numerous cells which could not be clearly
identified with one of the known islet cells
(Creutzfeldt et al., 1970).
Hazelwood and colleagues isolated a polypeptide from the pancreas of various species
(see Hazelwood, 1973) ; since it was first discovered in the extracts of chicken pancreas,
they called it "avian polypeptide" (APP).
It contains 36 amino acid residues and has
a molecular weight of 4200. As this substance is also found in the plasma of
chickens, it may possibly be a hormone.
Though APP is completely different from
gastrin, it strongly increases the secretion of
gastric acid and pepsin.
Fujita and co-workers (see Fujita and
Kaboyashi, 1971, 1973) induced granule release in the D-like cells of the pyloric antrum of the dog by lowering of the pH of
the stomach with 0.1 N hydrochloric acid.
Since this also causes the release of secretin,
it seems likely that these cells are the origin
of this hormone. Fujita and Kobayashi
(1973) consider it "highly probable" that
the pancreatic and gastrointestinal D-cells
are identical, which can only mean that
they consider secretin a third islet hormone.
Since the early days of experimental islet
research, the existence of a liver lipid-mohilizing islet hormone has been discussed
(Epple, 1965). The possibility that such a
substance is secreted by the D-cells received
strong support from the studies of Gabe
and Martoja (1969) on the annual cycle of
a hibernating mammal. In recent studies
on streptozotocin diabetic rats, we found
that the liver lipids were maintained at
normal levels for extended periods of time
despite a strong hyperlipemia and ketonuria, and it was concluded that there exists
a homeostatic control mechanism in the
absence of insulin (Epple, 1971). This control also exists in streptozotocin diabetic
rats after removal of the pituitary, the adrenals or the gonads (Epple and Car, 1971).
However, pancreatectomized rats will develop a fatty liver, even when fed exocrine
enzymes, unless insulin is injected (for
literature, see Chernick et al., 1972). These
observations suggest that a pancreatic hormone different from insulin controls the
liver lipids. Further studies must show if
this hormone is different from glucagon.
However, die lipolytic action of glucagon
in the adipose tissue should cause a fatty infiltration of the liver, and there seems to
be no proof that, under biological conditions, glucagon can control simultaneously both hepatic and adipose tissue lipids
(for literature, see Lefebvre, 1972). The observations of De Ova et al. (1971) in geese
"suggest that glucagon, in addition to its
577
COMPARATIVE ISLET HISTOPHYSIOLOGY
adipokinetic effect, decreases the release of
TGL by the liver into the circulation."
The problem of a specific liver lipid mobilizer is especially relevant for the control of the large amounts of liver lipids
in various lower vertebrates, which have
very little or no adipose tissue at all (see
Vague and Fenasse, 1965; Patent, 1970).
Amphiphil islet cells
These cells were first described in the islets of sharks, amphibians, and teleosts
(Epple, 1965; 1966a,6; 1967a,fo). Because of
their dual staining properties with the silver
method of Hellman and Hellerstrom (1960)
and with pseudoisocyanin after oxidation
(Schiebler and Schiessler, 1959), they were
termed "amphiphils." Since they are not
regular components of all islets and species
investigated, it was concluded that they
are not functionally independent. One case,
in which it could clearly be seen that the
argyrophil and pseudoisocyanin-methchromatic granules were not (or at least not
completely) identical (Epple, 1966a), suggests that they might be transitional types
between D- and B-cells.
Meanwhile, amphiphils have been confirmed by other authors for teleosts (Honma and Tamura, 1968), amphibians (Trandaburu and Ionescu, 1969), reptiles (Trandaburu and Calugareanu, 1969), and for
the human fetus (van Assche and Gepts,
1971), though the latter authors were obviously not aware of the findings in lower
vertebrates. Thus, amphiphils have been
found in all major groups of vertebrates,
except the cyclostomes and the birds.
The data presented here allow only the
conclusion that the amphiphils are phylogenetically widespread islet elements of uncertain significance. If it should turn out
that they are generally characteristic for
highly active islets as they are in the toad
(Epple, 1966a), they could become a valuable aid for the histophysiologist.
The pancreatic X-cells (holocephalian type)
Another type of argyrophil cell is represented by the X-cells of the holocephalians
(Fujita, 1962; Patent and Epple, 1967).
Since these cells are the most common type
of islet cell in the two species investigated,
and since their staining properties are completely different from those of the other
islet cells, it appears most likely that they
represent a fourth type of independent islet cell. Their size, shape, and capillary
supply (Patent and Epple, 1967) clearly attest to their secretory activity. No data
exist on the possible nature and role of
their hormone. However, it would be surprising if such an important islet component should only occur in one group of
vertebrates.
Recently, we found that the argyrophil
cells of the islet organ of the eel (Anguilla
rostrata) can be subdivided into two different groups: one, which shows the typical
characteristics of die D-cells, i.e., they are
also pseudoisocyanin-metachromatic and
fluorescent after HC1 hydrolysis (Solcia et
al., 1968), and another group, which is only
argyrophilic (Brinn and Epple, 1972,
1973). Since the A-cells of this species do not
stain with the Hellman and Hellerstrom
(1960) method, it is clear that we have
here a fourth cell type, a finding also supported by ultrastructural studies (Brinn,
1973). On morphological and topographical
grounds, it is easy to recognize two types
of argyrophils in the islets of another
teleost, Istiophorus platyptems (Hirano
and Honma, 1971). Similarly, the islets of
Tilapia mossarnbica contain more centrally
located argyrophils with a more radial orientation, and peripherally located, darker
argyrophils, with larger cell bodies and a
more tangential orientation (Epple, unpublished). It may well be possible that
one of these two fractions of argyrophils corresponds to the X-cells, but further studies
are urgently needed. For attempts to extract
a further islet hormone different from insulin and glucagon the pancreas of the
holocephalians should be ideal.
The
"li8hl"
c6lh
°f
lhe
The islets of both groups of cyclostomes,
the Petromyzonidae and the Myxiiiidae,
578
AUGUST EPPLE AND THOMAS L. LEWIS
have several unique topographical and his- showed nuclear enlargement and glycogen
tological features (see, e.g., Epple, 1969; infiltration. Alloxan destroys rather selecFalkmer and Patent, 1972; Falkmer et al., tively the B-cells (Winbladh Biuw, 1970).
]973a,b). For our consideration it is im- Agranular cells, probably the precursors of
portant that, not counting some uncommon both B- and "light" cells (see, e.g., Ermisch,
functional stages of the B-cells (Ermisch, 1966), are more abundant in the lobules
1966; Morris and Islam, 1969), only three containing "light" cells (Ermisch, 1966;
types of islet cells have been found, and Winbladh Biuw, 1970). According to van
that the A-cells are absent (see, e.g., Falk- Noorden et al., (1972), both light and dark
mer and Marques, 1972; Falkmer and Pat- islet cells contain ultrastructuraly granent, 1972; Falkmer et al., 1973&). There are ules, which confirms Ermisch's (1966) light
typical B-cells; "light" cells, which may or microscopical conclusions. Recently, we
may not be granular (Ermisch, 1966; Win- found mitoses in lobules which contained
bladh, 1966; Titlbach and Kern, 1969; van "light" cells, but no B-cells (Epple, unNoorden et al., 1972); and agranular cells published).
(Windbladh, 1966; Ermisch, 1966; Falkmer
It would indeed be a unique situation if
et al., 1973&).
the adult cyclostomes should have a rather
In recent years, the islet tissue of the constant number of hyperactive B-cells,
lampreys has been studied very extensively. largely separated from the normal B-cells.
There is a consensus of the investigators It seems that more experimental studies are
that the islets of younger specimens contain needed to finally clarify the nature of the
mainly B-cells, whereas in adults there are "light" cells. Perhaps it will be more fruit50% or more "light" islet cells (Ermisch, ful to look for differences between B- and
1966; Winbladh, 1966; Titlbach and Kern, "light" cells rather than for similarities,
1969). Ermisch (1966) was unable to decide keeping in mind that techniques applicable
if the "light" cells were functional stages in higher forms may have to be strongly
of the B-cells, or independent elements. modified in order to be useful in
Winbladh (1966) suggests that they may be cyclostomes.
the equivalents of the D-cells of higher
forms. Titlbach and Kern (1969) conclude
ISLET STRUCTURE VS. ISLET FUNCTION
that they are functional stages of the Bcells, since they observed ultrastructurally Quantitative islet changes
and with the aldehyde fuchsin stain tranThere seem to be no recent quantitative
sitional stages from typical B-cells to "light"
cells. If the "light" cells are functional measurements on changes of the islet volstages of the B-cells, then they must be ume of poikilothermic animals. In birds,
strongly stimulated B-cells, since their nu- there is a study on the changes of the islet
clei are larger (Ermisch, 1966), and the volume in growing chickens (Oakberg,
empty vesicles should indicate a high rate 1949). Most of the quantitative data available are on domestic or laboratory animals
of hormone release.
There are, however, some observations (Hoftiezer et al., 1971; for further literature,
which do not necessarily support the idea see Hellman, 1970; Saladino and Getty,
that the light cells are functional stages of 1972) and on the human (Ogilvie, 1964; see
the B-cells. Thus, B-cells and "light" cells also Hellman, 1970).
are largely, though by no means exclusively,
A correlation of the islet weight with the
located in different lobules. After pseudo- body weight should be rather difficult in
isocyanin stain with oxidation, there is a most species, mainly because of the great
clear separation between B- and "light" variations in the lipid deposits. Thus, for
cells (Ermisch, 1966). Ermisch (1966) ob- example, in the laboratory rat, the perviously did not observe transformations centages of liver and body lipids increase
after glucose injections, though the B-cells with age, maintaining a ratio of 1:3 (Epple,
579
COMPARATIVE ISLET HISTOPHYSIOLOGY
1971). In hibernating mammals, large
amounts of lipid are deposited in fall (for
literature, see Kayser, 1961; Hoffman,
1964). In birds, there are great seasonal
variations of the lipid stores in adipose
tissue, liver, and muscles (see, e.g., Farner
et al., 1968; Meier, 1972; Stetson and Erickson, 1972) depending on the migratory
habits of the species or subspecies. In
teleosts, it is very difficult to establish base
lines for organ:body weight ratios since,
according to Fontaine and Callamand
(1954), these fishes can be grouped into
four categories: (1) species with lean flesh
and small amounts of liver lipids; (2)
species with lean flesh and large amounts of
liver lipids; (3) fat-fleshed species with small
amounts of liver lipids; (4) fat-fleshed fishes
with large amounts of liver lipids and welldeveloped adipose tissue. On the other
hand, the emaciation of the anadromous
salmons is a well-known phenomenon, and
even sedentary trout populations may show
an annual cycle of the adipose tissue lipids
(Epple and Schneider, unpublished).
In birds, reptiles, amphibians, and teleosts, the seasonal changes of the gonadal
weight (especially the ovaries) add another
difficulty.
Thus, it is obvious that the most accurate
calculations of the total islet volume are
meaningless if they cannot be related to
useful parameters. This condition is hard
to meet in non-captive animals, especially
when different species are compared. More
than 30 years ago, Bargmann (1939) concluded that the available quantitative data
on the islet organ did not encourage any
conclusion concerning the biological role
of the islets. With respect to non-captive
animals, the situation is still the same.
Qualitative islet changes
Under non-pathological conditions, qualitative islet changes have been studied mainly in relation to aging and to events of the
annual cycle, such as hibernation and migration. The available data on seasonal
alterations have been covered by the reviews of Epple (1968a, 1969) and Falkmer
and Patent (1972). Concerning the functions of the islet hormones, only little can
be learned from these studies, mainly for
the following reasons: (1) The older studies
did not differentiate between A- and Bcells; (2) there are surprising species differences among birds (Epple and Farner,
1967; George and Naik, 1964) and among
teleosts (see Epple, 1969), which could not
yet be explained; (3) in hibernating mammals, the reports vary greatly (Falkmer and
Patent, 1972), and there are not sufficient
data to separate seasonal alterations from
those specifically related to the "winter
sleep"—if the latter type of alterations exists
at all (Pietschmann and Epple, unpublished).
Phylogenetic variations in islet structure
Since in most older studies the A- and Dcells have been lumped as "A"-cells, an attempt was made to list various groups of
vertebrates according to their percentage
of B-cells and X-cells. Table 2 shows the
larger systematic groups for which a general
statement may be permissible. Quantitative
data on other groups (chondrichthyes and
snakes) are found in the papers of Thomas
(1940, 1942), and most of the pertinent
literature has been compiled in the review
of Falkmer and Patent (1972).
The conclusions that can be drawn from
Table 2 are mainly negative: (1) The
TABLE 2. The predominant islet component among different taxa, for which a general statement
appears possible.
GROUP I
GROUP II
GROUP III
GROUP IV
B-cells only?
>50% B-cells
~50%X-eells
50% or >50% A-cells
Cyclostomes
Urodeles, mammals
Holocephalians
Lizards*
* Except for the Amphibaenidae.
Key references for this table: Bargmann (1939), Mosea (1959), Epple (1968a, 1969), Gabe
(1970), Miller and Lagios (1970), Falkmer and llarques (1972), Falkmer and Patent (1972).
580
AUGUST EPPLE AND THOMAS L. LEWIS
cyclostomes do not need pancreatic glucagon, but we do not know if this is really
a primitive feature, or how this is related
to their ways of life. The increase in "light"
cells in the islets of mature lampreys (Titlbach and Kern, 1969) cannot be explained
on the basis of data available. (2) The holocephalians are similar in their life habits
to certain elasmobranchs (see Patent, 1970),
which must have fewer, if any, X-cells, but
usually more B-cells (Thomas, 1940). (3)
Urodeles and mammals have a high percentage of B-cells, but there is no physiological or ecological explanation known.
Urodeles have very low, while mammals
have very high, blood sugar levels. Urodeles
do not depend on a minimum blood sugar
level (Copeland and De Roos, 1971), but
mammals do. Urodeles have little adipose
tissue (see Vague and Fenasse, 1965); mammals have large amounts of adipose tissue.
Urodeles are usually "carnivorous," while
the diet of m&mmals varies greatly. Urodeles
are either aquatic, terrestrial, or (in most
cases) both, while mammals are basically
terrestrial. (4) When the comparison of the
above criteria of urodeles and mammals
is extended to include teleosts, lizards, and
birds, no detailed accounting seems necessary to show that the percentage of B-cells
does not give us any clue as to the possible
role of insulin in these systematic groups
(see Table 3).
Thus, in accordance with the conclusions
of Miller (1961) and Gabe (1970) for the
reptiles and amphibians, we have to admit
that the present status of knowledge makes
it impossible to draw any conclusions from
the islet cell ratio as to the islet hormone
function.
Comparative data on the localization of
the islet cells within the islets of certain
species are given by Quay (1960), Aim and
Hellman (1964), Epple (1968*7), Gabe
(1970), and Hellman and Lernmark
(1970). Based on the situation in the
human, and supported by the stimulatory
effect of glucagon on insulin secretion in
vitro (see, e.g., Samols et al., 1972), there is
a wide-spread belief that the intermingling
of both A- and B-cells within the islets may
have a functional significance. However, on
a phylogenetical scale, there is a greater
tendency to separate the islet cell types than
to mix them. Grouping of the islet cells
within certain regions of the islets is typical
for the holocephalians (Patent and Epple,
1967), various amphibians (see, e.g., Epple,
\966a,b), various reptiles (Gabe 1970), and
birds (Clara, 1924a,b.). An intrainsular atrandom mixing of different islet cell types,
as found in the adult human, is not very
common; therefore, local interactions of
glucagon (see Samols et al., 1972) or the
hormone of the D-cells (Hellman and Lenmark, 1970) with the insulin secretion must
be a phylogenetically uncommon mechanism, if they play a role at all.
Islet composition and the effects of
pancreatectomy
In Table 3, the glycemia after total pancreatectomy is compared with the percentage of B-cells. Data on the various islet cytotoxins are not included, since we were unable to find any long-term observations in
lower vertebrates which clearly stated that
a practically complete destruction of one
type of islet cells was achieved without
serious damage to other organs (e.g., liver
and kidney). The results in Table 3 show
that total pancreatectomy is followed by a
permanent diabetes only in anurans, reptiles, and mammals studied so far. In the
cyclostomes, chondrichthyes, teleosts, and
birds, the results vary greatly and make a
general statement for the whole group impossible. However, it is clear that total pancreatectomy does not necessarily cause diabetes mellitus, and that, in general, the percentage of B-cells cannot be related to the
appearance of a permanent hyperglycemia.
CONCLUDING REMARKS
In keeping with the stated goal of this
review, we have emphasized unsolved problems of the islet histophysiology. The data
presented illustrate our ignorance, especially in three areas: (1) the phylogenetic
and ontogenetic origin of the endocrine
COMPARATIVE ISLET HISTOPHYSIOLOGY
581
pancreas: (2) the function of the established et al., 1971), the established term diabetes
islet hormones, insulin and glucagon, in mellitus may better be restricted to the
non-mammalian vertebrates; (3) the num- sequelae of insulin deficiency in mammals.
ber and nature of further islet hormones in
The mammalian bias may explain why it
all groups of vertebrates.
did not become clear until recently that the
Since we were unable to correlate the role of insulin in mammals may be a rather
islet structure with any major phylogeneti- special one. In mammals, nature invested
cal, ecological, or metabolic trend, we offer insulin with the control of practically all
the following hypothesis: The wide range anabolic processes. Therefore, removal of
of interactions between hypophysial and the pancreas causes in mammals a host of
islet hormones led to a phylogenetic com- metabolic disturbances which often make it
petition between both glands, whose out- extremely difficult to separate direct and income is reflected in the variations of the direct effects of insulin deficiency from
islet cytology. However, the nervous system those of the lack of other islet hormones.
also became involved. It gained a strong
The role of endogenous glucagon in
direct control of the islets in the teleosts "lower" vertebrates also requires an un(Klein and Lang, 1972; Brinn, 1973), and biased consideration, as shown by the folit lost its control of the islets completely in lowing examples: when in the toad after
birds (Kobayashi and Fujita, 1969; Tranda- hypophysectomy the blood sugar chops,
buru, 1972a,i>), being moderately successful the A-cells do not show any signs of inin mammals (Kobayashi and Fujita, 1969; creased activity (Epple et al., 1966); in the
Kern and Griibe, 1973).
snake, Natrix piscator hypophysectomy
It appears that the islet research in the causes a fatal hypoglycemia despite the
past was too strongly biased by the dramatic presence of liver glycogen (Rangneker and
effects of insulin deficiency in mammals, Padgaonkar, 1972). Obviously, in either
and as a consequence, most experimental case, glucagon did not counteract the drop
investigators in "lower" vertebrates looked of blood sugar; if it is involved in the
for the most obvious symptom of human maintenance of glycemia in these species at
all, its action must somehow depend on
diabetes, i.e., hyperglycemia.
This is especially well documented in the pituitary hormones.
research on the avian pancreas where sevThe study of islet hormones other than
eral publications since Gianelli's (1902) insulin may be more promising in lower
histological studies reported diabetes mel- forms as, e.g., the eel (Lewis and Epple,
litus as a result of pancreatectomy (for 1972) where pancreas removal is not folliterature, see Mialhe, 1958; Langslow and lowed by a dramatic breakdown of the
Hales, 1971). Now it seems that most of adipose tissue. However, despite the relthese investigators ignored a decisive his- atively low percentage of B-cells in teleosts
tophysiological phenomenon: the large and the widespread belief that all islet
amount of A-cells in the inconspicuous tissue in these fishes could be removed by
splenic lobe of the birds (Gianelli, 1902; simple isletectomy, no experiments on isClara, 1924a,£>). As shown by Mialhe and letectomized teleosts were published in the
co-workers (Mialhe, 1958, Sitbon, 1967), a 35 years between the papers of Simpson
truly total pancreatectomy causes—at least (1926) and the study of Falkmer (1961).
in ducks and geese—a hypoglycemic ten- Proinsulin was discovered in human tumors
dency rather than hyperglycemia; and since (see Steiner et al., 1969), and not in Brockavian adipose tissue is obviously insulin- mann bodies. Similarly, the hormonal
insensitive (Epple, 1961a; Goodridge and status of glucagon was disputed for thirty
Ball, 1966; Epple and Earner, 1967; Farner years, before the most logical source, the
et al., 1968), but very glucagon-sensitive large a-islets of the avian pancreas were
(Goodridge and Ball, 1965; Hoak et al., used for its extraction (Vuylsteke and De
1968; Grande and Prigge, 1970; De Oya Duve, 1953). Finally, in 1962, the unique
'Most prevalent"
(Thomas, 1940)
25%
(Falkmer, 1961; cf., Falkmer and Patent, 1972)
50%
(pers. observ.)
50%
(estimation from situation
in closely related Anguilla
rostrata)
66%
(v. Lawzewitsch, 1963a)
Raja erinacea
Pri'dominance of A-cells
(Miller, 1962)
Eumtcrs oosoletxts
Tupinamois rufcscens
Tupinambis tequixim
'only R-cells"
(Miller, 1961, 1962)
Tarirha torosa
liana pipiens
liana ligrina
Bufo d'oroignyi
Lt ptodactyliis crllatus
Ccratophrys ornata
Bufo arenamm
Anguilla anguilla
AnguiUa rostrata
Cottus scorpio
Mnstclus cants
Winbladh,
(Authors)
66%
(Falkmer and
1964)
56%
(Thomas, 1940)
fO -D*('ells
Myxinc glutinosa
Species
TABLE 3. The percentage
Falkmer (1961), Falkmer
and Matty (19666)
Hyperglycemia and glucosuria
Initial Hypoglycernia followed by hyperglycemia
Hypoglycemia
Hyperglycemia
Hyperglyeemia
Hyperglycemir.
Hyperglycemia.
Hyperglycemia.
Miller and Wurster (1958),
cf., Miller (1961)
Penhos et al. (1965)
Houssay and Biasotti (1930,
1933)
Houssay (1959)
Ragneker and Sabnis (1966)
Wurster and Miller (1960),
cf., Miller (1960)
Penhos and Krahl (1962);
v. Lawzewitsch (19636),
Penhos and Lavintman
(1964)
Caparelli (1894)
Grant et al. (1969)
Inconsistent glucosuria
Abramowitz et al. (1940)
Hypoglyeemia to Hyperglycemia with hyperglycemia
prevalent
Hypoglycemia
Lewis and Epple (1972)
Orias (1932)
Hyperglycemia
Inconsistent glycemia
Diamare (1906, 1911)
Schirner (1963), Falkmer
and Matty (1966a)
Authors
No clear results
None
Effect on blood sugar
Pancreatectomy/Isletectomy
of p-cells compared with the effects of total pancreateciomy.
Animals sacrificed after 2
days
Animals sacrificed after several weeks
Other islet cells may be
present in small numbers
(cf., Epple 19666)
Animals fasting
Only two days observation
time!
Methodical difficulties
Existence o£ cxlrainsular Bcells not excluded
Comments
55
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COMPARATIVE ISLET HISTOPHYSIOLOGY
topogi-aphical situation of the chicken pancreas was used to show the hyperglycemic
action of glucagon by removal of the third
and the splenic lobes (Mikami and Ono,
1962).
So far, it seems that no attempts have
been made to correlate the great regional
differences of the D-cells of the chicken
pancreas (Hellman and Hellerstrom, 1960;
Mikami and Mutoh, 1971) with the
amounts of extractable gastrin. Likewise, it
appears that there are no studies on the
possible occurrence of the hypokalemicdiarrheogenic factor of the Verner-Morrison
syndrome in the avian pancreas or in the
Brockmann bodies. No biochemical or
physiological studies on the hormone of the
X-cells of the holocephalians have been conducted since their discovery 10 years ago
(Fujita, 1962).
From the preceding it would appear that
future diabetes-related studies might greatly
benefit from the utilization of the results
obtained in lower vertebrates, while comparative islet research may be better off
when carried out free from the bias of the
mammalian situation.
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REFERENCES
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